blood transfusion
A horse can lose up to one-third of their blood before it becomes critical. “That’s around 13.5 liters (3.5 gallons) for the average-sized horse,” said Jamie Kopper, DVM, PhD, Dipl. ACVIM, ACVECC. “Beyond that, they’re likely going to need a blood transfusion to survive.”

Kopper, a large animal internist at Iowa State University’s College of Veterinary Medicine, in Ames, presented about acute anemia in horses at the 2021 Veterinary Meeting and Expo held in Orlando, Florida and virtually.

“In an acute hemorrhagic episode (an injury that results in significant blood loss), it can be difficult to gauge how much blood the horse has actually lost and whether anemia is going to be a problem,” Kopper said. “Horses can lose up to one-third of their blood before it becomes critical,” she explained.

Anemic crises can be tricky—the bleeding isn’t always evident, and transfusing in the field is no small feat. In this regard, Kopper shared practical tips for horse owners and veterinarians facing possible anemia.

What Can Cause Anemia?

Anemia, which is characterized as a loss of erythrocytes (red blood cells) in blood circulation, can have various causes:

External bleeding, generally brought on by a severe laceration or epistaxis (nosebleed) from guttural pouch mycosis.

Internal bleeding, which can present as hemoabdomen or hemothorax (blood in the abdomen or chest cavity, respectively) or as uterine artery rupture in the postpartum mare.

In these two scenarios, packed cell volume (PCV) and total protein (TP) are abnormally low, correlating with loss of whole blood (as opposed to loss of just erythrocytes). “But be prepared for a misleadingly normal PCV in the hours following the injury,” Kopper warned. “A splenic contraction triggered by significant blood loss can increase the number of circulating erythrocytes by up to 50%. Remember that a short-term erythrocyte boost doesn’t rule out impending anemia.”

Hemolysis (destruction of erythrocytes), which is generally immune-mediated and most commonly seen in neonatal isoerythrolysis cases. Other causes include equine infectious anemia (EIA) and oxidative injury to the erythrocytes caused by toxin ingestion or a drug reaction.

Decreased erythrocyte production, which occurs when bone marrow fails to release new red blood cells into circulation, though this condition is rare in horses.

“With these last two conditions, PCV will be low but the TP normal, because only the erythrocytes are lost, not whole blood,” Kopper said. “This important distinction can help pinpoint the source of the anemia.”

Life-Saving Blood Transfusions

Regardless of the cause, if a horse’s condition is critical enough, the veterinarian might decide to perform a blood transfusion. Lethargy, uncontrollable bleeding, inappetence for food or water, a heart rate above 60 beats per minute, and a respiratory rate above 32 breaths per minute are all indications the animal isn’t compensating well for the anemia. Kopper recommended considering transfusions for these patients. In addition to a suitable donor, the veterinarian will require a transfusion kit with fluid bags and lines, anticoagulants, and intravenous (IV) catheterization supplies for both donor and recipient for a successful transfusion. “Having a premade supplies kit will save precious time in an emergency situation,” said Kopper, who is a big advocate of preparation.

“The risks of a blood transfusion are nonnegligible,” she said. “We may get side effects, such as tachycardia, tachypnea, a fever, and/or hives, but when the blood transfusion is a live-saving attempt, these adverse reactions are generally the lesser of two evils.”

To minimize their occurrence, Kopper recommended choosing a donor that is, ideally, a large, healthy gelding who has not previously been transfused and starting the transfusion slowly while monitoring the recipient’s vital signs.

Take-Home Message

Luckily, not all anemia cases require transfusions, Kopper said. “In many cases, fluid administration and supportive care will sufficiently support the horse as he replenishes his erythrocyte-rich blood volume,” she said. “Close monitoring will help you decide when to pull the trigger and transfuse.”